#include <cstdio>
#include <fstream>
#include <cmath>

double mark_area(double a1, double a2, double b1, double b2)
{
	return (a1 * b2 - b1 * a2);
}

int main() 
{
	std::ifstream input("in.txt");
	std::ofstream output("out.txt");
	double x0, y0;
	double x1, y1;
	double x2, y2;
	double area = 0;

	input >> x0 >> y0;
	x1 = x0;
	y1 = y0;
	while (input >> x2 >> y2)
	{
		area += mark_area(x1, y1, x2, y2);
		x1 = x2;
		y1 = y2;
	}
	area += mark_area(x2, y2, x0, y0);
	area = fabs(area) * 0.5;
	output << std::fixed << area;	
	return 0;
}